The present invention relates to the drying of moist material in general, such as, bituminous and sub-bituminous coal, peat, wood, oil shale, ores, limestone and the like and, in particular, to a new and useful method and apparatus for preheating and drying such moist material using a rotary drum having heating tubes therethrough which are supplied with heated cooling gas from a dry coke cooling plant.
A coal mud drier is known from German Offenlegungsscrift No. 28 44 075, which is equipped with a hot gas drying system in a rotary drier furnace, in which a number of tubes is rigidly mounted in the longitudinal direction of the rotary drum of the furnace. Heating gases are continuously circulated through these tubes while waste gases, predominantly only enriched with steam, escape from the interior of the drum into the open atmosphere. To produce the heating gases, a separate burner is needed which must be operated with rich gases.
Such rotary drum driers are mostly disposed with a slight inclination and the moist fine material is fed in at the highest point of the drum while the preheated material is discharged at the lowest point. Proper structures are provided on the inside surface of the rotary drum by which the drying material migrating down between the heating tubes of the nests and accumulating on the drum bottom is lifted by the rotation of the drum and dropped again from above on the tube nests. With the slight inclination of the rotary drum, the fine material fed in at one end gradually passes to the other end where it is discharged. During its travel from the higher to the lower end, the fine material is repeatedly raised to the tube nests to flow down between the tubes and be dried and preheated.
To insure a trouble-free operation of such rotary drum driers, certain minimum requirements must be imposed on the charged material. For example, while drying coal, the moisture content of the charged coal must not affect the fluidity of the material charged to the tube nests. Otherwise, congestions and deposits of coal between the tubes and thus a reduced efficiency of the drier can be expected. In such cases, a remedy may be to enlarge the spacing between the tubes, for example, from 20 mm to 40 mm. However, this substantially reduces the heat exchange surface areas and thus again the efficiency of the drier. What is to be sought is rather to enlarge the heat exchange surface area by reducing the clearances for the passage of the fine material and thus to increase the drier efficiency. This particularly applies to the design of driers with a high throughput capacity.
Further difficulties arise during the operation of such drum driers with the removal of the produced vapors.
In these driers, the vapors are taken off toward the discharge end of the rotary drum through an opening in the top portion above the heating tube nest. Since for constructional reasons, this opening cannot be too large, the produced vapors entrain much fine dust and an expensive dust separation must be provided outside the rotary drier.